The extracellular space in the developing rat brain: Its variation with changes in osmolarity of the fixative, method of fixation and maturation

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Considerable extracellular space was demonstrated in the newborn rat cerebral cortex. The extent of this space could be easily modified by changes in the osmolarity of the glutaraldehyde fixative. With maturation this space became progressively less, generally from the deeper layers outward, and attained adult dimensions by the end of the second week. However, in the white matter, between subpial astrocytes, between ependymal cells and between the cells of the choroid plexus, much narrower spaces were present at all ages. These results compare well with the morphologic findings reported by others following primary glutaraldehyde fixation or freeze-substitution, as well as with the physiologic measurements of the cerebral extracellular spaces in the immature brain also reported by other investigators. Following immersion fixation, the extracellular space was somewhat narrower. The most marked decrease in the extracellular space was noted after immersion fixation in isotonic fixative. All these observations of a relatively large extracellular space in the immature brain contrast with the narrow space seen following primary osmium tetroxide fixation reported by other investigators. It is concluded that the cerebral cortical extracellular space in the immature brain is quite large, but that its true in vivo extent cannot be definitely demonstrated because of the sensitivity of the immature brain to the processes of fixation and embedding.